Phosphonium ionic liquids as lubricants for aluminium-steel

Abstract

The performance of a series of novel room temperature ionic liquids (ILs) based on the trihexyl(tetradecyl)phosphoniumcation (P 66614 + ) and a number of novel anions have been studied in pin-on-disk tests using a 100Cr6 steel ball on AA2024 aluminium disks. The anions coupled to the (P 66614 + ) cation include diphenyl phosphate (DPP-), dibutyl phosphate (DBP-), bis (2,4,4-trimethyl pentyl) phosphinate (M 3 PPh -) and bis(2-ethyl hexyl) phosphate (BEH-). More traditional anions such as bis(trifluoromethanesulfonyl) amide (NTf 2 -) and bromide (Br-) were also investigated.Experiments were conducted at various loads to assess the IL film forming abilities. The results suggest that the structure of the anion is important in forming a surface film that reduces the friction and wear of the aluminium disk. At 30N five of the six ILs tested showed a 30-90% reduction in wear, as determined from wear scar depth measurements, compared to fully formulated diesel oil. The IL lubricant with a diphenyl phosphate anion achieved the lowest wear coefficient, showing a better performance than a typical fluorine-containing IL anion, NTf 2 . To further investigate wear mechanisms and surface interactions the wear scars were analysed using a scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS).

A Review of Ionic Liquid Lubricants

“…Since this report, research has focused on more specific systems, for example, the steel on steel system has had the majority of the interest [16][17][18][19][20][21][22][23][24][25][26]. The more challenging systems in terms of efficient lubrication, including steel/aluminium and titanium/steel, have also received attention [7,15,[27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42].…”

“…Other non-fluorinated anions that include known useful elements, such as phosphorus (e.g., dibutylphosphate, DBP) and boron (e.g., bis(oxalato)borate, BOB) have also been studied. In the wear environment, all of these anions were assumed to undergo favourable reactions to result in protective tribofilms [22,23,25,[36][37][38][39].…”

“…As the cation chemistry available has expanded, so too has the type of cations being investigated in IL lubricants. In subsequent work, ILs with an ammonium cation were investigated as an alternative to imidazoliums [7,24,27], followed by the quaternary phosphonium cation [21,33,36,46]. Interestingly, the pyrrolidinium family of cations has only received a little attention thus far, even though they have shown some promising wear performance results [24,38].…”

“…In an effort to improve the protective tribofilm formation, phosphate anions were chosen to mimic the traditional anti-wear additives, such as zinc dialkyl dithiophosphate (ZDDP) [22,23,25,36,45]. Anions, such as dimethylphosphate and diphenylphosphate, have been investigated and have shown similar or lower wear than NTf 2 and FAP for both the steel/steel and aluminium/steel systems [22,38].…”

“…X-Ray photoelectron spectroscopy (XPS) has been the most widely used, due to its ability to identify not only the elements present on the wear scar surface, but also the chemical state of the atom [6,[15][16][17][18][19][20][21][22][23]25,[28][29][30]32,33,35,38,40,44,47,48,[51][52][53][54]. Scanning electron microscopy (SEM) has been used to identify the type of wear occurring based on the wear scars appearance with the associated energy dispersive X-ray spectroscopy (EDX) used to ascertain the elements present on the wear scar [15,17,[19][20][21]23,25,[28][29][30][31][32][33][34][35][36][37][40][41][42][43]…”